1. Field of the Invention
This invention relates to a method for preparing rare earth compounds. More particularly, the invention relates to the preparation of rare earth carbonates and, subsequently, conversion of the carbonates into high-density rare earth oxides.
2. Description of the Art
Rare earth compounds, once only scientific curiosities, are finding ever increasing industrial uses. By far the heaviest use of rare earth compounds is in the production of catalysts for organic reactions, particularly for the cracking of crude petroleum. Rare earth compounds are also used in the glass industry as polishing agents, refractive index modifiers, and colorants, as alloying agents in the metallurgical industry, in high purity forms for phosphors in color television picture tubes, and more recently, in the production of intermetallic compositions for the manufacture of high energy permanent magnets.
In particular, alloys of samarium and cobalt, having the compositions SmCo.sub.5 and Sm.sub.2 Co.sub.17, produce commercially useful permanent magnets having very high energy products. Samarium for these compositions is generally produced from samarium oxide by reduction at high temperatures. In one method, the samarium is reduced with an active rare earth metal, such as cerium, lanthanum, or misch metal, under a very high vacuum; elemental samarium sublimes, is collected, and later is alloyed with elemental cobalt in the proper proportions. Alternatively, the proper mixture of samarium oxide and cobalt oxide is reduced with calcium metal under an inert atmosphere to directly produce the alloy material.
Different users of rare earth compounds have different specifications for the properties of the materials which they purchase. For example, a producer of samarium-cobalt intermetallics will have productivity constrained by the volume capacity of the production equipment. By packing a larger quantity of samarium oxide into process vessels, a higher throughput can be obtained in a production run. Thus, samarium oxide having a high bulk density is very desirable.
Rare earth minerals typically contain a mixture of many different rare earth elements; solutions obtained from dissolving the rare earth portion of the ores in aqueous acids are subjected to liquid-liquid extraction procedures to separate the individual rare earths. The resulting rare earth solutions typically are treated with appropriate reagents for precipitating rare earth oxalates, carbonates, or hydroxides for recovery of the individual rare earth. The precipitate is then calcined at a temperature between about 900.degree. C. and about 1,500.degree. C., producing rare earth oxide.
Samarium oxide derived from samarium carbonate precipitated under non-specific conditions typically has a low bulk density near 1.0 grams/cc. The samarium oxide derived from samarium oxalate typically has an intermediate bulk density near 1.5 grams/cc, which is obtained at significantly higher reagent costs. However, there remains a need, particularly in alloy production, for a samarium oxide of even higher density specifications, preferably above 2.0 grams/cc. Of course, samarium oxide producers have a continuing need to reduce their operating costs.
Of the reagents commonly used for precipitating rare earths, oxalates (including oxalic acid) generally yield the most easily recovered precipitates. A substantial reduction in production costs could be achieved by precipitating rare earths with a less costly reagent, such as a carbonate or bicarbonate. However, as discussed by M. M. Woyski and R. E. Harris, "The Rare Earths and Rare Earth Compounds" in I. M. Kolthoff and P. J. Elving (Ed.). Treatise on Analytical Chemistry, Part II, Volume 8, John Wiley and Sons, Inc., New York, 1963, at page 16, the normal precipitation technique for rare earth carbonates yields slimy, relatively unfilterable solids.
In view of the foregoing, it is an object of this invention to provide a method for preparing high-density rare earth oxides.
It is a further object of the invention to provide a method for preparing high-density rare earth oxides, using relatively inexpensive reagents.
A further object of the invention is to provide a method for precipitating rare earth carbonates which can readily be separated from the mother liquor.
These and other objects will appear to those skilled in the art, from consideration of the following description and claims.